UNIVERSITY   Of   CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY.  CALIFORNIA 


PLUM  POLLINATION 


BY 

A.  H.  HENDRICKSON 


BULLETIN  No.  310 

July,  1919 


UNIVERSITY   OF  CALIFORNIA  PRESS 

BERKELEY 

1919 


EXPERIMENT  STATION  STAFF 


HEADS   OF  DIVISIONS 


Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Walter  Mulford,  Forestry,  Director  of  Eesident  Instruction. 

Herbert  J.  Webber,  Director  Agricultural  Experiment  Station. 

B.  H.  C-  ocheron,  Director  of  Agricultural  Extension. 
Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

James  T.  Barrett,  Acting  Director  of  Citrus  Experiment  Station;  Plant  Path- 
ology. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Charles  W.  Woodworth,  Entomology. 
Ralph  E.  Smith,  Plant  Pathology. 
J.  Eliot  Coit,  Citriculture. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Enology. 
Warren  T.  Clarke,  Agricultural  Extension. 
John  S.  Burd,  Agricultural  Chemistry. 
Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 
Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
J.  0.  Whitten,  Pomology. 

Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 

F.  L.  Griffin,  Agricultural  Education. 
John  E.  Dougherty,  Poultry  Husbandry. 
S.  S.  Rogers,  Olericulture. 
L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 


Division  of  Pomology 

J.  C.  Whitten  W.  L.  Howard 

A.  H.  Hendrickson  W.  P.  Tufts 

E.  L.  Overholser  G.  L.  Philp 

R.  M.  Amesbury  M.  N.  Wood 

H.  Sevier 


PLUM  POLLINATION 

By  A.  H.  HENDRICKSON 


Pollination  experiments  with  plums  and  prunes  have  been  carried 
on  by  the  Division  of  Pomology  of  the  University  of  California  for 
five  years.  The  work  was  done  chiefly  at  the  University  Farm  at 
Davis  and  in  the  Santa  Clara  Valley.  Later,  several  varieties  were 
experimented  upon  at  Vacaville.  These  experiments  have  yielded 
much  definite  information  regarding  the  necessity  for  cross-pollinating 
the  different  varieties  to  insure  a  satisfactory  set  of  fruit. 

Only  two  species  of  plums  are  of  much  commercial  importance  in 
California — namely,  the  Japanese  (Primus  tri flora,  Roxbg.)  and  the 
European  (Primus  domestioa,  Linn.).  The  former  consists  entirely 
of  shipping  plums,  the  chief  varieties  being  Climax,  Wickson,  Kelsey, 
Burbank,  Santa  Rosa,  Beauty,  and  Abundance.  Of  the  European 
varieties,  Tragedy,  Clyman,  Grand  Duke,  and  Pond  are  used  exten- 
sively for  shipping,  and  the  French  (Agen  or  Petite)  Imperial,  Sugar, 
and  Robe  de  Sergeant  are  used  principally  for  drying. 

In  certain  sections  of  the  state,  extensive  areas  are  devoted  to  the 
growing  of  plums  and  prunes,  and  often  large  plantings  consist  of 
but  one  or  two  varieties.  The  State  Commission  of  Horticulture  has 
compiled  figures  which  show  that  in  the  Santa  Clara  Valley  alone 
there  are  over  61,000  acres  planted  to  prunes,  and  that  this  acreage 
is  largely  planted  to  one  variety,  the  French,  with  a  few  scattered 
plantings  of  Imperial  and  Sugar.  Yields  of  many  varieties  of  plums 
and  prunes  have  not  always  been  satisfactory  and  growers  suspected 
the  trouble  might  be  due  to  improper  pollination.  Many  complaints 
of  this  kind  have  reached  the  University  both  by  private  communi- 
cation and  through  the  public  press.  Most  of  these  complaints  have 
come  from  the  growers  of  shipping  plums,  many  of  which  are  uncer- 
tain bearers.  Such  varieties  as  Tragedy,  Pond,  Satsuma,  Wickson 
and  others  have  been  frequently  reported  in  the  agricultural  papers 
as  erratic  in  productiveness. 

With  prunes,  the  lack  of  fruitfulness  generally  did  not  so 
definitely  point  to  lack  of  proper  pollination.  While  the  Imperial, 
Robe  de  Sergeant,  and  Silver  prunes  have  long  been  considered  shy 
bearers,  the  French  and  the  Sugar  have  continued  to  bear  more  or 
less  satisfactorily  when  set  out  in  large  blocks  of  a  single  variety. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Many  growers  in  the  Santa  Clara  Valley  have  noticed  an  appreciable 
falling  off  in  their  crops,  and  have  been  anxious  for  information 
relative  to  the  cause.  Improper  pollination,  due  to  self-sterility,  was 
believed  to  be  at  least  one  important  factor  causing  this  imperfect 
set.  Another  factor  has  already  been  shown  to  be  responsible  to  a 
degree  for  this  failure  to  set  commercial  crops.  This  factor  is  the  lack 
of  honey  bees  in  the  orchards  at  blooming  time.  Proper  pollination 
has  been  shown  to  be  dependent  upon  insect  agencies  to  carry  the 
pollen.1  It  is  probable  that  other  factors  such  as  pruning,  cultivation, 
fertilization  and  irrigation  will  be  shown  to  influence  the  amount  of 
fruit  set. 

OBJECTS    OF   THE    INVESTIGATION 

It  was  with  these  problems  in  view  that  the  Division  of  Pomology 
in  the  spring  of  1914  began  an  investigation  of  the  pollination  question 
concerning  the  leading  varieties  of  plums  and  prunes  in  California. 
The  experiments  were  designed  to  answer  the  following  questions : 

1.  What  varieties  are  self-sterile  (requiring  cross-pollination)  ? 

2.  What  varieties  are  self -fertile    (not  requiring  cross-pollin- 

ation) ? 

3.  What  varieties,  although  not  self-sterile,  may  be  benefited 

by  cross-pollination  ? 

4.  What  are  the  best  pollinators  for  the  various  self-sterile 

varieties  ? 

5.  What  is  the  relation  of  weather  conditions  to  the  set  of  fruit, 

open  to  pollination  by  natural  agencies? 

One  of  the  main  objects  was  to  find  out  if  possible  what  varieties 
were  able  to  set  fruit  through  the  influence  of  their  own  pollen. 
Among  the  stone  fruits,  the  peach  has  long  been  considered  self- 
fertile,  but  Waugh  and  others  have  shown  that  most  of  the  native 
American  plums  and  many  of  the  European  plums  require  cross- 
pollination.  More  recently  Gardner  has  shown  that  most  varieties 
of  sweet  cherries  are  self -sterile,  and,  furthermore,  certain  varieties 
were  inter-sterile  as  well.  Tufts  of  California  has  verified  Gardner's 
results  with  sweet  cherries  and  also  found  that  the  same  facts  apply  to 
certain  varieties  of  almonds.  The  question  as  to  the  ability  of  varie- 
ties of  plums  to  set  and  mature  crops  through  the  medium  of  their 
own  pollen  is  of  utmost  importance  to  the  grower.  Moreover,  the 
problem  of  affinity  between  varieties  must  be  considered  in  order  to 
recommend  the  proper  varieties  for  interplanting. 

1  Cal.  Agr.  Exp.  Sta.  Bulls.  274  and  291,  The  Common  Honey  Bee  as  an  Agent 
in  Prune  Pollination   (1916,  1918). 


PLUM    POLLINATION  5 

Considerable  progress  was  made  upon  other  problems  such  as  the 
effect  of  crossing  upon  the  size  and  color  of  fruit,  blossoming  periods, 
amount  of  pollen  produced,  arrangement  of  trees  necessary  for 
cross-pollinating  purposes,  and  other  problems  closely  allied  with  the 
foregoing.  In  some  cases  definite  information  was  obtained,  while 
in  others  only  general  statements  as  a  result  of  long  continued  obser- 
vation can  be  made. 

OUTLINE    OF    EXPERIMENTS 

Location. — Most  of  the  work  was  done  at  the  University  Farm  at 
Davis,  in  the  Sacramento  Valley  about  thirteen  miles  west  of  Sacra- 
mento. The  orchard  was  about  six  years  old  when  the  experiments 
were  started  and  consisted  of  a  collection  of  nine  varieties  of  Japanese 
and  fourteen  varieties  of  European  plums.  All  trees  were  in  a  healthy, 
vigorous  condition  and  had  been  pruned  annually  by  thinning  out 
branches  where  too  thick  and  by  cutting  off  about  two-thirds  of  the 
previous  season's  growth.  In  the  Santa  Clara  Valley,  where  experi- 
ments were  started  in  1915,  the  trees  used  were  twelve  years  old  or 
older  and  had  been  in  bearing  for  a  number  of  years.  In  most  cases 
these  trees  appeared  less  vigorous  than  those  at  the  University  Farm. 

How  the  Work  was  Done. — With  each  variety  the  chief  emphasis 
was  placed  upon  the  question  as  to  whether  or  not  it  was  self-fertile, 
in  the  sense  of  its  being  able  to  set  and  mature  fruit  when  self- 
pollinated,  regardless  of  whether  a  viable  seed  was  produced  or  not. 
The  first  season  (1914)  blossoms  of  the  varieties  being  tested  were 
enclosed  in  paper  bags  (fig.  1)  while  still  in  the  bud,  after  the  method 
used  by  "Waugh,  and  allowed  to  open  and  pollinate  themselves  while 
thus  protected  from  insect  visits.  During  the  following  year  and  all 
succeeding  seasons,  however,  all  blossoms  thus  enclosed  were  actually 
pollinated  with  pollen  from  the  same  variety.  In  this  way  lack  of  set 
would  more  fully  indicate  self -sterility  and  all  doubts  as  to  whether 
the  stigma  actually  received  pollen  would  be  eliminated. 

Crosses  were  made  on  each  variety,  using  a  number  of  other 
varieties  of  the  same  species  that  blossomed  at  approximately  the 
same  time.  Varieties  used  in  crossing  were,  as  far  as  possible,  com- 
mercial sorts  adapted  to  the  same  general  conditions  as  the  female 
parent.     No  inter-species  crosses  were  made. 

With  both  the  self-pollinations  and  the  crosses  the  blossoms  were 
emasculated  by  inserting  the  nails  of  the  thumb  and  second  finger 
through  the  calyx  tube  slightly  below  the  point  where  the  petals  are 
attached,  and  then  by  a  deft  jerk  removing  petals  and  stamens  with 
one  movement.     The  ovarj^  and  style  were  seldom  if  ever  injured. 


6 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


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Fig.  1. — Robo  de  Sergeant  prune  tree,  showing  blooming  habit.  Note  paper 
bags  used  for  covering  flower  clusters  in  the  pollination  experiments.  University 
Farm  Orchard,   Davis. 


PLUM    POLLINATION  7 

Furthermore,  any  slight  degree  of  mutilation  to  the  flower  was  the 
same  for  the  self-pollinations  and  the  crosses. 

Several  thousand  blossoms  on  different  branches  among  the  several 
trees  were  counted  each  year  and  left  open  to  natural  pollination  by 
insects.  Fruits  resulting  from  these  blossoms  were  counted  three 
times  during  the  season..  The  number  of  fruits  secured  by  this  natural 
pollination  served  as  a  check  on  the  main  experiments  and  are  referred 
to  hereafter  as  the  ' '  normal  set. ' '  This  count  also  constituted  a  record 
of  the  varying  set  from  year  to  year  so  it  would  be  possible  to  find 
what  percentage  of  set  constituted  a  satisfactory  commercial  crop. 

The  bags  were  removed  from  the  self -pollinated  and  crossed  fruits 
about  three  weeks  after  pollination,  and  the  number  of  fruits  set  were 
counted  at  this  time.  They  were  counted  a  second  time  early  in  May 
and  again  before  picking  in  order  to  observe  their  behavior  throughout 
the  season. 

Except  during  the  year  1914,  no  pollen  was  used  until  samples 
had  been  subjected  to  artificial  germination  tests.2 

The  early  varieties,  particularly  the  Japanese  sorts,  did  not  pro- 
duce a  plentiful  supply  of  pollen  and  it  was  often  difficult  to  secure 
viable  pollen  in  time  for  use  when  the  pistils  were  receptive.  Pistils, 
if  left  for  more  than  six  days  after  emasculation,  before  pollen  was 
supplied,  usually  gave  a  poor  set  of  fruit.  Furthermore,  the  varieties 
of  plums  used,  did  not  yield  better  pollen  when  twigs  were  brought 
into  the  greenhouse  and  forced  into  bloom.  This  method,  used  by 
Lewis  and  Vincent  with  apples,  did  not  work  out  successfully  with 
plums. 

EXPERIMENTAL    RESULTS 

Seasons  of  1914-17  Inclusive,  University  Farm  and  Santa  Clara 
Valley. — During  the  season  of  1914  five  varieties  of  Japanese  plums 
and  four  varieties  of  European  plums  were  experimented  upon.  The 
Japanese  varieties  were  the  Combination,  Satsuma,  Kelsey,  Wickson, 
and  Burbank.  Two  of  the  European  type  were  Clyman  and  Tragedy, 
and  the  other  two,  French  and  Robe  de  Sergeant  prunes.  Between 
1000  and  1500  blossoms  were  counted  upon  each  variety  to  determine 
the  normal  set.  From  900  to  1300  blossoms  of  each  were  self-pollinated 
and  from  150  to  600  blossoms  of  each  were  crossed  with  the  leading 
varieties  in  blossom  at  the  same  time.    The  work  of  emasculating  and 


2  These  tests  are  not  included  in  this  bulletin  for  lack  of  space,  but  are  on  file 
in  the  office  of  the  Division  of  Pomology,  where  they  may  be  consulted  by  those 
interested.  The  details  of  testing  pollen,  and  also  the  collection  and  care  of  the 
pollen,  as  well  as  its  transferance  to  the  flowers  in  the  orchard,  will  be  fully 
described  in  a  following  paper. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

pollinating  was  done  between  February  25  and  March  30.     Records 
show  the  season  to  have  been  normal  during  the  blossoming  period. 

In  1915  the  work  was  carried  on  in  a  way  similar  to  that  done 
in  1914,  except  that  all  self-pollinations  were  actually  made  by  hand 
to  make  sure  each  stigma  received  its  supply  of  pollen.  This  method 
was  used  in  all  subsequent  seasons.  Climax  and  Sugar  were  included 
in  the  list  of  varieties  in  the  experiment.  The  work,  furthermore, 
was  broadened  to  include  investigations  on  the  French,  Sugar,  and 
Imperial  prunes  in  the  Santa  Clara  Valley,  the  foremost  prune  pro- 
ducing section  in  California.  Opportunity  was  given  at  the  latter 
place  for  experimenting  with  mature  trees  of  the  type  (fig.  2)  gen- 
erally conceded  to  be  gradually  falling  off  in  production.  Emascula- 
tions and  pollinations  at  Davis  were  continued  from  March  3  to 
March  22.  The  weather  during  the  early  part  of  the  season  was 
moderately  cool  but  later  became  unseasonably  warm.  This  con- 
dition was  especially  noticeable  in  the  Santa  Clara  Valley  where  the 
mean  temperature  for  the  month  was  2.9°  F.  above  normal.  No 
precipitation  was  recorded.  Considerable  complaint  was  heard  at 
the  latter  place  because  of  the  failure  of  the  prunes  to  set  a  full  crop 
and  the  cause  was  popularly  attributed  to  the  unusually  warm  weather 
at  blossoming  time.  Results  obtained  at  Davis  generally  substantiated 
those  of  the  previous  year.  In  the  Santa  Clara  Valley  the  French 
and  Sugar  prunes  showed  a  tendency  toward  self -fertility,  while  the 
Imperial  pointed  to  self-sterility. 

In  1916  the  chief  aim  was  to  reduce  experimental  error  as  much 
as  possible  by  increasing  the  number  of  blossoms  used.  The  seasonal 
climatic  conditions  were  approximately  normal  in  both  districts.  At 
Davis  the  work  was  done  between  February  24  and  March  18,  while 
in  the  Santa  Clara  Valley  the  season  was  practically  the  same  as  that 
for  the  previous  year.  A  strong  north  wind  at  Davis  on  one  occasion 
blew  a  number  of  bags  from  the  trees,  injuring  a  number  of  blossoms 
under  experiment.  The  results  obtained  at  Davis  were  practically 
the  same  as  for  the  previous  seasons.  In  the  Santa  Clara  Valley 
using  large  numbers  of  blossoms,  it  was  decisively  demonstrated  that 
the  three  varieties  of  prunes  grown  there  are  well  able  to  effectively 
cross-pollinate  one  another.  Especially  satisfactory  results  were 
obtained  on  the  Sorosis  ranch  by  crossing  the  French  prune  with  the 
Imperial  (fig.  3). 

Two  additional  Japanese  plums,  Sultan  and  Abundance,  and 
three  additional  European  plums,  Grand  Duke,  Pond,  and  Yellow 
Egg,  were  used  in  3917.  The  weather  during  the  early  part  of  the 
season  at  Davis  was  unusually  cold  and  the  blossoms  opened  slowly,  but 


PLUM    POLLINATION 


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Fig.  2. — Old  trees  of  French  prune  in  the  Santa  Clara  Valley.     This  variety 
blooms  very  profusely — much  more  so  than  the  Sugar  prune. 


10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


later,  climatic  conditions  ameliorated  and  a  succession  of  bright  days 
gave  ample  opportunity  for  insect  pollination.  The  later  blooming 
varieties,  consistent  with  previous  seasons'  results,  set  heavier  than 
the  early  ones  which  open  when  the  weather  is  still  cold.  In  the  Santa 
Clara  Valley  the  season  was  one  which  brought  the  blossoms  along 
slowly.  A  slight  frost  occurred  nearly  every  night  in  the  early  part 
of  the  season,  but  no  damage  to  the  blossoms  resulted.  The  work  in 
the  latter  section  was  done  between  March  17  and  March  24.  The 
results  coincided  with  those  obtained  in  previous  years,  and  consider- 
able weight  must  be  given  to  the  results  in  the  Santa  Clara  Valley 


Pig.  3. — The  two  small  branches  of  fruits  shown  are  French  X  Imperial.    These 
are  typical  of  the  results  secured  from  crossing  the  French  and  Imperial  in  1916. 


as  they  were  obtained  from  using  3000  to  5000  blossoms  for  each 
cross,  all  of  which  were  hand  pollinated.  Results  from  both  sections 
are  fully  discussed  later  in  this  paper. 

Season  of  1918:  University  Farm,  Santa  Clara  Valley,  and  Vaca 
Valley. — The  spring  of  1918  was  markedly  unfavorable  for  pollination 
work  on  account  of  frosts,  rain  and  high  winds.  At  Davis  rain  fell 
on  March  10,  11,  and  12  and  again  during  the  week  beginning 
March  16.  A  high  wind  on  March  18  blew  many  bags  from  the  trees, 
injuring  many  blossoms  enclosed  for  experimental  purposes.  Finally, 
two  killing  frosts  on  the  nights  of  April  4  and  5  followed  two  days  of 


PLUM   POLLINATION  11 

high  north  winds.  It  was  noticed  on  April  5  that  many  of  the  pistils 
of  the  French  prune  were  killed  and  it  is  probable  that  blossoms  on 
other  varieties  hand-pollinated  a  few  days  earlier  were  similarly 
injured.  In  addition  to  the  foregoing  difficulties  there  was  a  light 
bloom  on  many  varieties,  probably  due  to  the  dry  autumn  and  winter 
preceding.  It  was  therefore  impossible  to  experiment  upon  Burbank, 
Satsuma,  Kelsey,  Abundance,  Clyman,  Yellow  Egg,  Pond,  and  Sugar. 

As  a  result  of  this  scarcity  of  blossoms  at  the  University  Farm, 
arrangements  were  made  to  continue  the  work  with  Japanese  varieties 
on  the  farm  of  Mr.  Millard  Sharpe  of  Vacaville.  Several  new  varieties 
— the  Beauty,  Formosa,  Gaviota  and  Santa  Rosa,  were  thus  added 
to  the  experimental  list.  However,  frost  on  the  nights  of  April  4 
and  5  caused  considerable  damage  to  fruit  buds  at  Vacaville,  the  same 
as  it  did  at  Davis.  As  a  consequence  the  only  positive  results  obtained 
were  with  the  Wickson,  all  other  crosses  and  self-pollinations  with  the 
Japanese  sorts  being  lost.  The  Wickson  gave  the  following  percent- 
ages of  set  with  the  various  crosses :  Satsuma,  6.6  per  cent ;  Kelsey, 
2.8  per  cent ;  Beauty,  4.6  per  cent ;  Formosa,  6.4  per  cent ;  and 
Santa  Rosa,  9.9  per  cent.  These  results  indicate  that  Wickson  can  be 
successfully  cross-pollinated  by  Beauty,  Formosa  and  Santa  Rosa  in 
addition  to  the  varieties  previously  noted. 

In  general,  the  results  at  Davis  with  the  few  varieties  which  were 
uninjured  by  the  frost  were  the  same  as  for  the  preceding  four  seasons. 
Bloom  on  all  varieties  was  scanty,  and  therefore  in  spite  of  a  high 
percentage  of  set  in  some  cases,  the  resulting  yields  did  not  constitute 
a  heavy  crop.  In  the  Santa  Clara  Valley  identical  results  with  those 
of  the  three  preceding  years'  work  were  obtained.  The  normal  set  of 
the  French  and  Imperial  prunes  in  the  Sorosis  orchard,  however, 
showed  a  decided^  lower  percentage  than  in  1917.  This  decrease 
was  probably  due  to  the  exceptionally  heavy  crop  produced  by  this 
orchard  in  1917. 3 

PRESENTATION    OF   DATA 

The  essential  data  obtained  during  the  five  years  are  herewith 
presented  in  table  1.  With  the  idea  in  view  of  presenting  to  the 
grower  in  an  easily  understandable  form  the  essential  results  obtained 
from  the  work  on  the  pollination  of  plums  and  prunes,  only  the  total 
number  of  blossoms  used,  the  average  per  cent  of  bloom  that  set  fruit, 
the  number  of  years  averaged  and  the  average  yield  per  tree  are 
given.  Likewise  a  full  discussion  is  given  summarizing  the  results 
as  a  whole  rather  than  taking  up  each  year's  results  separately. 


s  For  full  account  see  Calif.  Agr.  Exp.  Sta.,  Bull.  291,  The  Common  Honey  Bee 
as  an  Agent  in  Prune  Pollination. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  1 

Summary  of  the  Normal  Set  and  Self-  and  Cross-Pollination  of  Plums  and 

Prunes  for  the  Years  1914-1918 


Location 
Variety  and  of 

Treatment  Orchard 

ComLination 

Normal  Set Davis 

XSelf 

XKelsey 

XSatsuma 

XWickson 

Kelsey 

Normal  Set Davis 

XSelf 

X  Combination... 

XSatsuma 

XBurbank 

XWickson 

Satsuma 

Normal  Set Davis 

XSelf 

X Combination..  . 

XKelsey 

XBurbank 

XWickson 

Burbank 

Normal  Set Davis 

XSelf 

XKelsey 

XSatsuma 

XWickson 

XClimax 

X  Sultan 

Wickson 

Normal  Set Davis 

XSelf 

XKelsey 

XSatsuma 

XBurbank 

XClimax 

XSultan 

XAbundance 

Climax 

Normal  Set Davis 

XSelf 

XKelsey 

XBurbank 

XWickson 

XAbundance 


Total 

No.  of 

blossoms 

used 

Av.  per 
cent,  of 

bloom 
that  set 

fruit 

No.  of 
years 
aver- 
aged 

Av.  yield 
per  tree 
for  1915, 
1916  &  17                        Notes 

5926 

3.5 

4 

42 

1914-17 

2450 

0 

4 

1914-17 

58 

0 

1 

1911 

1788 

1.6 

4 

1914-17 

1213 

0.75 

2 

1916-17 

4744 

9.0 

3 

38.1 

1915-17  frost  1918 

1240 

0 

4 

1914-18  frost  1918 

54 

0 

1 

1914 

589 

1.5 

2 

1914  and  1917 

465 

0.55 

2 

1916-17  frost  1918 

772 

2.6 

3 

1915-17  frost  1918 

5094 

3.9 

3 

34.8 

1914,  15,  17  frost  1918 

2414 

0 

3 

1914,  15,  17  frost  1918 

423 

1.4 

2 

1914,  15 

173 

0 

2 

1914  and  17 

335 

0.9 

1 

1917 

484 

6.4 

1 

1917  frost  1918 

6123 

25.6 

4 

85 

1914-17  frost  1918 

2957 

0 

4 

1914-17  frost  1918 

833 

1.2 

2 

1915,  16  frost  1918 

905 

17.5 

2 

1914,  17  frost  1918 

1470 

15.0 

4 

1914,  17  frost  1918 

685 

15.1 

2 

1914,  17  frost  1918 

169 

18.0 

1 

1916,  17  frost  1918 

7301 

4.8 

4 

106 

1914-17  frost  1918 

3582 

0 

4 

1914-17  frost  1918 

921 

1.8 

3 

1914-16  frost  1918 

1663 

6.1 

4 

1914-17  frost  1918 

1412 

3.6 

3 

1914,  16,  17,  frost  1918 

794 

2.6 

2 

1916,  17 

34 

0 

1 

1917 

230 

1.3 

1 

191/ 

6938 

3.5 

3 

99 

1915-17,  frost  1918 

1247 

2.8 

3 

1915-17,  frost  1918 

510 

0.3 

3 

1915-17,  frost  1918 

107) 

4.6 

3 

1915-17,  frost  1918 

1215 

2.5 

3 

1915-17,  frost  1918 

441 

4.6 

1 

1917,  frost  1918 

PLUM   POLLINATION 


13 


Table  1 — (Continued) 


Location 
Variety  and  of 

Treatment  Orchard 

Sultan 

Normal  Set Davis 

XSelf 

XKelsey 

XBurbank 

X  Wickson 

Abundance 

Normal  Set Davis 

XSelf 

XBurbank 

X  Wick?  on 

XClimax 

Clyman 

Normal  Set Davis 

XSelf 

XTragedy 

X  Yellow  Egg 

XPond  

X Grand  Duke.... 
Tragedy 

Normal  Set Davis 

XSelf 

XClyman 

X  Yellow  Egg 

XPond 

X Grand  Duke  .... 

XRobe  de 

Sergeant 

XSugar 

Yellow  Egg 

Normal  Set Davis 

XSelf 

XTragedy 

XPond 

X Grand  Duke  .... 
Pond 

Normal  Set Davis 

XSelf 

X  Yellow  Egg 

X Grand  Duke  .... 
Grand  Duke 

Normal  Set Davis 

XSelf 

XClyman 

X  Tragedy 

X  Yellow  Egg 

XPond.. 

2One  year. 


Total 

No.  of 

blossoms 

used 


2693 
300 
140 
283 
394 

2338 
635 
383 
427 
353 

6005 

1126 

509 

159 

37 

76 

6270 
3679 
2507 
517 
513 
1235 

100 
34 

880 
301 
239 
283 
299 

1205 
306 

268 
268 

1965 
684 
320 
696 
309 
395 


Av.  per 

cent,  of 

bloom 

that  set 

fruit 


4.0 
0 

1.9 

9.1 

12.2 

24.1 
0.1 
5.4 

15.4 
9.9 

25.9 

0 
18.1 

1.2 
13.5 
10.3 

20.9 
0.02 
14.7 

8.75 
0.75 
4.4 

27.7 
2.9 

58.7 
9.3 

8.7 
18.3 

5.7 

41.7 
4.2 

21.3 
7.5 

31.5 
0.01 
6.9 
0 

0.3 
O.i 


No.  of  Av.  yield 

years  per  tree 

aver-  for  1915, 

aged  1916  &  17  Notes 

481  1917,  frost  1918 
1917,  frost  1918 
1917,  frost  1918 
1917,  frost  1918 
1917,  frost  1918 

1261  1917,  frost  1918 
1917,  frost  1918 
1917,  frost  1918 
1917,  frost  1918 
1917,  frost  1918 

4  36.7     1914-17 

2  1916,  17 

3  1914,  16,  17 
1  long  interval  1917 
1  1917 

1  1917 

5  118    1914-18 
5         1914-18 

4  1914-17 

2  1916  long  interval  1917 
2 

3  1916,  17,  frost  1918 

1914 
1915 

381  191 7 
1917 
1917 
1917 
1917 

601  1917 
1917 
1917 
1917 

2  1351        1917,  18,  frost  1918 

2  1917,  18,  frost  1918 

1  1917 

2  1917,  18,  frost  1918 
1  1917 
I  1917 


14 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 


Table  1 — (Continued) 


Total 
Location         No.  of 

Variety  and  of             blossoms 

Treatment  Orchard           used 

French 

Normal  Set Davis          8109 

XSelf 2743 

X  Sugar 1341 

XRobe  de 

Sergeant 1756 

Sugar 

Normal  Set Davis          1919 

XSelf 417 

XFrench 332 

XRobe  de 

Sergeant 121 

Robe  de  Sergeant 

Normal  Set Davis          6204 

XSelf 2754 

XFrench 1780 

XSugar 1272 

XTragedy 579 

XPond 153 

X Imperial  Gage..  215 

French 

Normal  Set Cupertino  20939 

X  Normal  Set 14845 

XSelf 2948 

Xlmperial 2156 

XSugar 2530 

French3 

Normal  Set Saratoga     23661 

Normal  Set4 18774 

XSelf 5052 

Xlmperial 6195 

XSugar 6022 

Imperial 

Normal  Set Cupertino     6529 

XSelf 1083 

XFrench 635 

XSugar 819 

Imperial 

Normal  Set Saratoga       7315 

XSelf 4010 

XFrench 5039 

XSugar 4783 

Sugar 

Normal  Set Cupertino     7299 

XSelf 1077 

XFrench 1054 

Xlmperial 993 

2No  weights  taken.         3Next  to  Imperials. 


Av.  per 

cent,  of 

bloom 

that  set 

fruit 

No.  of 
years 
aver- 
aged 

Av.  yield 

per  tree 

for  1915, 

1916  &  17 

Notes 

33.0 

5 

81.4     1914- 

-18, 

frost  1918 

3.02 

5 

1914- 

-18, 

frost  1918 

10.6 

4 

1914- 

17 

6.6 


0.8 


1914-17 


41.6 

2 

88.7     1915,17 

8.1 

2 

1915,  17 

2.7 

1 

1917 

1917 


19.7 

5 

91.4     1914-18, 

frost  injury 

'18 

0.16 

5 

1914-18, 

frost  injury 

'18 

10.5 

5 

1914-18, 

frost  injury 

'18 

13.2 

4 

1914-17 

20.4 

2 

1914,  17 

45.1 

1 

1914 

20.0 

1 

1914 

4.0 

4 

2      1915-18 

4.4 

4 

1915-18 

9.6 

4 

1915-18 

23.5 

4 

1915-18 

21.1 

4 

1915-18 

7.6 

4 

2      1915-18 

6.5 

4 

1915-18 

5.05 

4 

1915-18 

10.9 

4 

1915-18 

22.8 

3 

1916-18 

3.5 

3 

2      1915-17 

0.3 

3 

1915-17 

1.3 

3 

1915-17 

6.0 

3 

1915-17 

7.25 

4 

2      1915-18 

1.5 

4 

1915-18 

8.7 

4 

1915-18 

5.0 

3 

1916-18 

11.7 

4 

2      1915-18 

8.4 

4 

1915-18 

13.9 

4 

1915-18 

11.1 

4 

1915-18 

4Five  rows  from  Imperials. 


PLUM    POLLINATION  15 

DISCUSSION   OF   RESULTS 

Japanese  Varieties. — By  a  study  of  table  1  it  will  be  noticed  that 
the  average  set  of  fruit  for  the  Japanese  varieties  under  ordinary 
orchard  conditions  prevailing  at  the  University  Farm,  was  approxi- 
mately 4  per  cent.  Only  in  a  few  cases  is  there  a  noticeable  deviation 
from  this  average.  Kelsey  on  a  three-year  average  set  9  per  cent, 
and  Abundance,  in  the  one-year  counts  made  on  that  variety,  set  24.1 
per  cent.  For  four  years  the  Burbank  has  maintained  the  remarkably 
high  average  of  25.6  per  cent.  Invariably  this  variety  set  fruit  so 
thickly  that  a  heavy  thinning  was  necessary  to  bring  the  fruit  up  to 
marketable  size.  All  varieties  of  Japanese  plums  were  given  the 
same  kind  of  care  and  pruned  in  the  same  way,  consequently  the 
percentage  of  fruit  set  by  each  should  be  considered  a  varietal  char- 
acteristic. The  heavv  set  obtained  with  the  Burbank  often  necessitates 
thinning.  Other  varieties,  notably  the  Wickson  and  Climax,  matured 
satisfactory  commercial  crops  without  the  added  expense  of  the  heavy 
thinning  which  was  necessary  in  the  case  of  the  Burbank. 

The  yield  records  for  these  trees  which  are  available  for  the  years 
1915,  1916,  and  1917,  show  some  further  interesting  facts.  The  first 
three  varieties,  which  as  a  rule  are  the  first  to  blossom  each  spring, 
show  a  comparatively  low  yield  record.  Combination  yielded  at  the 
average  rate  of  83,  24,  and  20  pounds  per  tree  for  the  three  years, 
giving  an  average  of  42  pounds  per  tree  resulting  from  a  set  of  3.5 
per  cent.  Kelsey  gave  a  consistent  average  yield  of  46,  38.4,  and  30 
pounds,  or  a  total  average  of  38.1  pounds  for  three  years  from  an 
average  set  of  9  per  cent.  No  great  difference  in  yield  followed  in 
1917  as  the  result  of  the  low  percentage  of  set.  Satsuma  produced 
an  average  yield  per  tree  of  40.4  and  60  pounds,  or  a  total  average 
of  50.2  for  the  years  1915  and  1917.  These  yields  resulted  from  an 
average  normal  set  of  3.9  per  cent.  The  foregoing  varieties  blossom 
very  early  and  the  yields  thus  far  can  only  be  considered  fair  from 
a  commercial  point  of  view. 

Burbank  produced  an  average  yield  per  tree  of  74.5,  108,  and  73 
pounds,  giving  a  total  average  of  85  pounds  per  tree,  as  a  result  of  the 
continued  high  average  set  of  25.6  per  cent.  While  this  crop  was 
entirely  satisfactory,  it  must  be  compared  with  the  next  two  given 
below  to  arrive  at  the  value  of  the  percentage  of  set  as  a  determining 
factor  in  the  amount  of  fruit  produced.  Wickson  matured  an  average 
per  tree  of  84,  89,  and  147  pounds,  or  a  total  average  of  106  pounds 
per  tree  as  a  result  of  the  comparatively  low  set  of  4.8  per  cent. 
Climax  responded  similarly  to  Wickson.    It  set  an  average  of  82,  83.4, 


16  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

and  132  pounds  per  tree  or  a  total  average  of  99.1  pounds  per  tree 
for  three  years,  resulting  from  an  average  set  of  3.5  per  cent.  Sultan 
showed  during  one  year  some  of  the  characteristics  of  Climax  and 
Wickson  by  maturing  a  set  of  4  per  cent  or  48  pounds  of  fruit  per 
tree.  Abundance  on  the  other  hand  matured  a  set  of  24.1  per  cent 
or  126  pounds  per  tree. 

The  most  noticeable  result  of  the  past  four  season's  work,  with 
the  Japanese  varieties,  excluding  the  results  with  most  varieties  dur- 
ing 1918  because  of  frost  injury,  is  the  distinctness  with  which  they 
fall  into  one  of  two  classes — those  which  are  self-sterile  and  those 
which  are  self-fertile.  Only  one  variety,  the  Climax,  out  of  the  eight 
varieties  experimented  with  is  self-fertile.  All  the  others  never,  or 
practically  never,  set  fruit  when  self -pollinated. 

In  the  field  method  of  experimentation  with  which  these  experi- 
ments were  carried  out,  no  morphological  defects  in  any  of  the  flowers 
were  detected.  Some  of  the  blossoms  of  the  Wickson  variety  produced 
very  short  pistils  that  measured  approximately  one-fourth  the  length 
of  a  normal  pistil.  These,  however,  were  never  used.  The  factor  of 
self-sterility  as  far  as  could  be  shown  was  simply  due  to  the  inability 
of  the  several  varieties  to  set  fruit  with  their  own  pollen.  In  making 
the  counts  after  blooming  time  there  never  was  any  doubt  as  to  what 
varieties  were  self-sterile.  The  self-pollinated  pistils  turned  yellow 
and  dropped  soon  after  the  pollen  was  applied.  Often  blossoms  open 
to  insect  pollination,  and  which  dropped  off  later,  still  retained  their 
floral  parts  and  were  firmly  attached  to  the  limb,  when  the  self- 
pollinated  ones  had  withered  and  fallen.  Sometimes  a  few  fruits 
persisted  until  the  second  count,  but  all  usually  fell  before  the  fruit 
ripened. 

As  a  result  of  four  years '  experimentation  in  crossing  the  Japanese 
plums  with  one  another,  one  fact  stands  out  with  remarkable  con- 
sistency, which  is  that  whenever  crosses  were  made  using  the  pollen 
of  Combination  and  Kelsey  the  set  of  fruit  was  low.  The  cause  cannot 
be  stated  with  certainty  but  in  the  opinion  of  the  writer  it  is  probably 
due  to  inability  to  secure  strong  vigorous  pollen  from  these  varieties. 
Their  natural  production  is  usually  scanty,  and  being  formed  while 
the  weather  is  still  cold  it  is  often  necessary  to  wait  until  late  in 
the  season  to  secure  a  supply.  Even  then,  the  germination  tests  rank 
among  the  lowest  of  any  of  the  varieties  used.  The  low  results  from 
using  the  pollen  of  these  varieties  even  on  some  of  the  later  more 
vigorous  ones,  agree  with  the  low  average  yield  of  these  varieties, 
inasmuch  as  they  must,  for  the  most  part,  be  inter-pollinated  through 
the  agency  of  insects,  or  from  the  very  early  blossoms  of  the  later 


PLUM    POLLINATION  17 

blooming  varieties.  Combination  and  Kelsey  must  be  considered  as 
inferior  pollenizers  not  only  among  themselves  but  for  some  of  the 
other  later  varieties.  Furthermore,  it  has  not  been  possible  to  obtain 
a  satisfactory  set  by  crossing  artificially,  when  either  the  Combination 
or  Kelsey  were  used  as  the  female  parent. 

The  Satsuma  has  been  successful  as  a  pollenizer  for  several  of 
the  early  and  mid-season  blooming  kinds.  This  was  especially  true 
when  either  the  Burbank  or  Wickson  were  used  as  the  female  parent. 
No  variety  has  proved  particularly  valuable  as  a  cross-pollenizer  for 
the  Satsuma,  though  pollinated  by  the  Wickson,  one  season  it  pro- 
duced a  set  of  6.4  per  cent.  Since  the  Satsuma  is  known  as  a  shy 
bearer  in  many  sections  of  the  state  it  is  unfortunate  that  a  satisfac- 
tory pollenizer  of  it  was  not  found. 

Results  with  Burbank  indicate  that  while  absolutely  self-sterile 
it  is  readily  pollinated  by  any  one  of  the  several  varieties  used,  and 
is  also  an  efficient  pollenizer  for  the  other  varieties  which  blossom  at 
the  same  time.  The  only  poor  results  were  obtained  when  Kelsey  was 
used  as  the  male  parent.  The  reciprocal  of  this  cross,  viz.,  Kelsey  X 
Burbank,  also  gave  low  results  both  years  it  was  tried.  Crossed  with 
Satsuma,  Wickson,  Climax,  or  Sultan,  Burbank  gave  as  satisfactory 
results  as  could  have  been  desired.  The  reciprocals  of  these  crosses 
were  also  satisfactory.  Burbank  is  undoubtedly  one  of  the  most 
efficient  pollenizers  of  its  season,  and  a  valuable  variety  for  this  reason. 

The  Wickson  was  also  found  to  be  a  reliable  cropper  and  an 
excellent  pollenizer.  When  Wickson  was  used  as  the  female  parent 
all  pollenizers  gave  satisfactory  results  except  Kelsey,  Sultan,  and 
Abundance.  Kelsey  has  already  been  mentioned  as  an  unreliable 
pollenizer.  The  Sultan  and  Abundance  crosses  were  only  made  in 
1917  and  as  the  interval  of  time  between  emasculation  and  pollination 
was  eleven  and  twelve  days,  respectively,  these  two  varieties  cannot 
be  eliminated  as  pollenizers  for  the  Wickson  without  further  trial. 
Wickson  has  proved  its  worth  as  a  pollenizer  for  Satsuma,  Burbank, 
Climax,  Sultan,  and  Abundance  by  the  high  average  set  these  crosses 
produced  throughout  a  series  of  trials.  With  Burbank  it  ranks  as 
one  of  the  best  pollenizers. 

Climax  in  addition  to  being  self-fertile  was  found  to  be  easily 
pollenized  with  Burbank,  Wickson,  and  Abundance.  With  Kelsey 
as  the  male  parent,  no  better  results  were  obtained  than  when  Kelsey 
was  used  with  other  varieties.  Climax  pollenized  Burbank,  Wickson, 
and  Abundance  satisfactorily.  Because  of  its  tendency  toward  self- 
fertility,  and  because  it  is  a  good  pollenizer,  Climax  is  a  valuable 
variety.  Furthermore,  it  blossoms  rather  late  in  the  season,  thus 
tending  to  escape  late  spring  frosts. 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Although  tried  out  but  one  year  Sultan  and  Abundance  gave 
evidence  of  being  useful  as  pollenizers.  They  also  gave  high  sets  when 
crossed  with  Burbank,  Wickson,  and  Climax.  Because  of  their  late 
blossoming  they  should  be  valuable  to  interplant  with  Burbank,  Wick- 
son,  or  Climax. 

European  Shipping  Plums. — The  tendency  has  been  with  the 
young  trees  of  Clyman  and  Tragedy  varieties  on  the  University  Farm 
to  increase  year  by  year  the  percentage  of  fruit  set.  With  each 
increase  in  percentage  of  fruit  set,  has  come  an  increase  in  yield. 
Thus  the  trend  of  both  the  percentage  of  set  and  the  yield  has  been 
upward.  Records  also  show  that  the  Yellow  Egg,  Pond,  and  Grand 
Duke  varieties  have  steadily  increased  in  yield  year  by  year,  but  no 
records  were  kept  of  the  percentage  of  blossoms  which  set  fruit  until 
1917,  so  no  comparison  can  be  made.  These  European  varieties  set 
a  higher  percentage  of  blossoms  than  do  the  Japanese  varieties,  but 
have  not  as  yet  produced  a  markedly  larger  crop. 

Clyman  produced  an  average  of  22.5,  27.7,  and  60  pounds  per 
tree,  or  an  average  for  the  three  years  of  36.7  pounds  from  an  average 
set  of  25.9  per  cent.  This  average  can  only  be  considered  a  fair  com- 
mercial crop.  Coupled  with  the  fact  that  a  high  percentage  of  set 
produces  only  a  small  commercial  crop,  it  would  seem  as  if  this  variety 
did  not  produce  enough  blossoms  to  make  it  a  success ;  it  would  in 
fact  probably  soon  be  discarded,  if  it  were  not  for  its  value  as  a 
pollenizer  of  the  Tragedy.  It  has  a  reputation  for  light  bearing  and 
the  fruit  is  not  above  mediocre  in  quality  for  shipping. 

The  Tragedy,  on  the  other  hand,  has  matured  an  average  per 
tree  of  55,  85.6,  and  215  pounds,  or  an  average  set  of  23.7  per  cent 
for  four  years.  Thus  with  this  variety  a  high  set  has  given  a  good 
commercial  crop.  An  average  production  of  118  pounds  per  tree  from 
the  Tragedy,  as  compared  with  36  pounds  per  tree  from  the  Clyman, 
with  the  percentage  of  set  practically  the  same,  would  seem  to  indi- 
cate that  the  Tragedy  was  much  more  prolific  in  the  production  of 
blossoms.  Furthermore,  it  produces  a  great  many  fruit  spurs,  and 
the  fruit  itself  is  of  such  quality  that  it  is  a  general  favorite.  Yellow 
Egg,  Pond,  and  Grand  Duke  all  show  evidence  of  maturing  a  com- 
paratively high  percentage  of  set. 

The  self-sterility  of  the  two  varieties  experimented  upon  for  four 
years  was  well  established.  There  is  no  question,  either  from  growers 
or  from  experimental  data  at  hand,  but  that  Clyman  and  Tragedy 
must  be  cross-pollinated  to  produce  satisfactory  crops.  Grand  Duke 
in  one  year's  experiments  also  seems  self-sterile.  Yellow  Egg  and 
Pond  both  set  a  small  percentage  of  fruit  when  self -pollinated,  show- 


PLUM   POLLINATION  19 

ing  that  not  as  large  a  proportion  of  European  plums  are  self-sterile 
as  was  the  case  with  the  Japanese  varieties. 

Decisive  results  have  shown  the  best  pollenizers  for  at  least  some 
of  the  five  varieties  thus  far  experimented  upon.  Clyman  was  satis- 
factorily pollinated  with  Tragedy,  Pond,  and  Grand  Duke.  The 
average  set  of  Clyman  crossed  with  Tragedy  for  three  years  was  17.1 
per  cent.  While  Yellow  Egg  did  not  give  a  high  result  when  used 
with  Clyman,  this  fact  cannot  be  construed  to  mean  it  will  not 
pollenize  the  latter,  as  an  eleven-day  interval  elapsed  between  emascu- 
lation and  pollination.  In  turn,  Clyman  for  four  years  has  been  a 
highly  efficient  pollenizer  for  Tragedy,  as  shown  by  the  average  set 
of  14.7.  Clyman  also  pollinated  Grand  Duke  effectively.  Despite  its 
serious  drawbacks,  there  is  no  question  as  to  the  value  of  Clyman  as 
a  pollinator  of  the  Tragedy.  It  might  even  be  good  commercial  prac- 
tice to  interplant  Tragedies  with  a  few  Clymans  solely  for  the  benefit 
of  the  former. 

For  the  valuable  and  important  variety,  Tragedy,  a  number  of 
the  European  varieties  of  plums  seem  to  be  efficient  pollenizers.  Chief 
among  these  is  the  Clyman.  Next  in  importance,  and  blossoming 
with  it  at  Davis,  is  Grand  Duke.  In  three  seasons  this  cross  gave  an 
average  set  of  4.4  per  cent.  Robe  de  Sergeant  in  1914  crossed  on 
Tragedy  gave  a  set  of  27.7  per  cent  which,  while  high,  probably  is 
of  no  commercial  importance  because  of  the  fact  that  these  two  varie- 
ties are  not  adapted  to  being  grown  in  the  same  districts.  As  with 
some  other  varieties,  the  crosses  of  Yellow  Egg  and  Pond  on  Tragedy, 
the  only  year  they  were  tried,  were  made  so  late  that  the  pistils  were 
not  in  good  physical  condition.  As  a  pollenizer  Tragedy  does  well 
both  on  Clyman  and  Grand  Duke,  so  that  it  might  be  interplanted 
with  either  of  these  varieties  to  advantage. 

The  Grand  Duke,  Yellow  Egg,  and  Pond  inter-pollinated  success- 
fully in  1917  when  the  pollen  was  applied  before  the  pistils  began  to 
wither.  In  general  they  have  proved  to  be  excellent  pollenizers  not 
only  for  themselves,  but  for  the  Clyman  and  Tragedy  also.  As  these 
latter  varieties  have  been  used  for  practically  only  one  year  their 
value  cannot  be  considered  as  authoritative  as  that  of  varieties  tried 
out  for  four  seasons.  All  of  the  European  shipping  plums  which  have 
come  under  observation  have  proved  to  be  abundant  pollen  producers. 
Furthermore,  the  pollen  is  easily  germinated  and  generally  shows  a 
high  percentage  of  viability. 

Prunes. — The  three  varieties  of  prunes,  French,  Sugar,  and  Robe 
de  Sergeant,  experimented  upon  at  the  University  Farm  have  set 
comparatively  high  percentages  of  fruit  and  matured  fairly  satis- 


20 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


factory  crops  considering  the  age  of  the  trees.  The  French  and  Robe 
de  Sergeant  yielded  consistently  each  year,  but  the  Sugars  failed  to 
blossom  in  1916.  As  this  failure  followed  a  heavy  crop  it  was  thought 
that  the  drain  upon  the  tree  had  been  so  great  that  it  was  unable 
to  mature  fruit  buds  for  the  following  season.  This  variety  again  in 
1918  behaved  in  the'  same  manner,  failing  to  produce  any  blossoms. 
This  seeming  tendency  towards  alternate  bearing  is  an  uncommon 
occurrence  with  this  variety.  The  general  reports  concerning  it  are 
that  it  bears  heavily  and  regularly.     However,  from  four  years'  data 


Fig.  4. — The  characteristic  shape  of  the  French  prune  was  not  influenced  by 
being  pollinated  by  other  varieties.  In  this  case  the  crosses  were  made  with 
Sugar  and  Imperial. 


at  Davis,  it  would  seem  that  if  young  trees  just  coming  into  profitable 
bearing  are  allowed  to  overbear  one  year,  only  a  light  crop  is  pro- 
duced the  following  season.  One  of  the  most  serious  criticisms  of  the 
Sugar  prune  is  that  it  tends  to  overbear. 

The  French  and  Robe  de  Sergeant  at  Davis,  on  the  basis  of 
regularity  in  bearing,  have  proved  to  be  more  valuable  than  the  Sugar. 
The  former  variety  during  the  three  years  of  1915,  1916,  and  1917 
produced  an  average  crop  of  57.5,  41.7,  and  145  pounds,  respectively, 
or  an  average  of  81.4  pounds  per  year  for  the  three  years.  This  crop 
resulted  from  an  average  set  of  28.7  per  cent.  The  Robe  de  Sergeant 
practically  equaled  the  record  of  the  French.     In  three  years  the 


PLUM    POLLINATION  21 

average  crop  per  tree  was  95.1,  79.6,  and  100.6  pounds,  or  a  total 
average  per  tree  of  91.4  pounds  from  an  average  set  of  17.4  per  cent. 
Roughly  speaking,  the  above  yields  approximate  three  tons  of  green 
fruit  per  acre,  which  is  a  fairly  satisfactory  commercial  yield  consider- 
ing the  age  of  the  trees  (eight  years  in  1915).  In  the  case  of  the 
Sugar  prune  the  average  yield  is  only  available  for  two  years,  as  no 
crop  at  all  was  produced  in  1916.  These  yields  were  30.1  and  147.3 
pounds  per  tree,  or  an  average  of  88.7  pounds  per  tree  for  the  two 
years,  resulting  from  the  high  average  percentage  of  set  of  41.6  per 
cent.  If  this  yield  is  calculated  on  the  basis  of  three  years,  as  might 
properly  be  done,  the  average  yield  for  the  variety  is  thus  reduced  to 
but  59.1  pounds  per  tree. 

Two  varieties  of  these  prunes,  the  French  and  Sugar,  are  self- 
fertile,  while  the  third,  Robe  de  Sergeant,  is  self -sterile.  Four  years' 
results  with  self -pollinating  the  French  prunes  have  shown  great 
fluctuation  in  the  percentage  of  fruit  obtained.  Some  years  this 
percentage  was  high  and  in  others  it  was  low,  but  the  average  for 
four  years  shows  that  this  variety  must  be  accepted  as  self-fertile. 
Upon  what  factors  this  self -fertility  depends  is  not  known  but  it  seems 
to  be  largerly  a  matter  of  applying  the  pollen  at  exactly  the  right 
time.  Climatic  conditions  following  the  application  of  pollen  may 
also  exert  some  influence  on  the  resultant  set.  This  conclusion  as  to 
the  self-fertility  of  the  French  prune,  which  is  emphasized  by  the 
Santa  Clara  experiments,  is  of  utmost  importance  to  the  prune 
growers  of  California.  It  shows  why  vast  acreages  of  this  variety 
have  been  able  to  produce  crops  year  after  year  without  the  necessity 
of  growing  fillers  as  pollenizers.  The  next  problem  is  to  find  what 
environmental  conditions  favor  the  highest  percentage  of  set  when 
the  blossoms  are  pollinated  with  their  own  pollen,  as  must  necessarily 
be  the  case  in  most  of  our  prune-growing  sections. 

The  Sugar  prune  which  is  supposedly  a  seedling  of  the  French 
showed  an  average  set  of  8.1  per  cent  from  self -pollinated  flowers. 
This  evidence  was  corroborated  by  data  obtained  in  the  Santa  Clara 
Valley  (four-year  average,  8.4  per  cent)  and  shows  there  is  no  need 
of  interplanting  other  varieties  with  the  Sugar  for  purposes  of  cross- 
pollination.  The  Robe  de  Sergeant  has  emphatically  and  consistently 
proved  to  be  self-sterile  and  in  urgent  need  of  cross-pollination  to 
secure  crops.  These  data  support  the  widespread  contention  among 
growers  that  unless  it  is  properly  interplanted  with  other  varieties, 
the  Robe  de  Sergeant  is  a  shy  bearer. 

In  spite  of  their  reputed  relationship  no  trouble  was  experienced 
during  the  four  years  in  obtaining  satisfactory  sets  when  French  was 


22 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


crossed  with  Sugar  or  vice  versa.  The  French  prune  was  found  to 
be  readily  cross-pollinated  by  either  the  Sugar  or  Robe  de  Sergeant. 
Although  the  average  result  of  these  hand-made  crosses  was  not  as 
high  as  the  set  under  open  orchard  conditions,  they  were  higher  than 
the  average  set  resulting  from  the  self-pollinations.  Thus  it  was 
shown  that,  even  though  self-fertile,  the  French  prune  might  still 
be  benefited  by  pollen  other  than  its  own.  The  French  prune  has  the 
additional  good  quality  of  being  a  heavy  pollen  producer  and  an 
excellent  pollenizer  for  the  other  varieties  of  prunes.     The  French 


Fig.  5. — Shows  results  of  pollinating  the  Imperial  with  pollen  from  the  Sugar 
and  French  varieties.     The  typical  shape  of  the  Imperial  is  retained  by  the  fruits. 


as  a  pollenizer  for  the  Robe  de  Sergeant  gave  an  average  of  10.5  per 
cent  set  for  the  five-year  period.  The  one  year 's  results  as  a  pollenizer 
for  the  Sugar  cannot  be  considered  infallible  as  results  in  the  Santa 
Clara  Valley  have  proved  the  French  an  efficient  pollenizer  for  the 
Sugar.  The  probable  causes  of  the  low  set  for  this  particular  cross 
are  discussed  elsewhere  (p.  20). 

No  eminently  noticeable  results  were  obtained  at  Davis  when 
Sugar  was  used  as  the  female  parent.  Due  to  various  causes  the 
crosses  shown  in  the  table  were  only  tried  once.  As  a  pollenizer  for 
both  the  French  and  Robe  de  Sergeant,  the  Sugar  has  no  equal.  It 
produces  an  abundance  of  pollen,  blossoms  at  practically  the  same 


PLUM    POLLINATION  23 

time,  and  has  proved  effective  in  four  years'  trials.  The  high  per- 
centage (10.6  per  cent)  when  used  on  French,  and  the  still  higher 
percentage  (13.2  per  cent)  when  used  on  Robe  de  Sergeant,  amply 
attest  its  value. 

While  Robe  de  Sergeant  was  proved  to  be  self -sterile,  it  was  also 
shown  to  be  readily  capable  of  fertilization  by  any  other  European 
plum  blossoming  at  the  same  time.  French  and  Sugar  as  pollenizers 
for  four  years  gave  the  best  average  results,  but  Tragedy,  Pond,  and 
Imperial  Gage,  tried  for  a  lesser  number  of  seasons  also  show  possibili- 
ties as  pollenizers  for  this  variety.  It  is  perhaps  important  to  note 
that  the  prunes  commonly  grown  with  the  Robe  de  Sergeant  are  so 
efficient  as  pollenizers  that  the  other  varieties  mentioned  (Tragedy, 
Pond,  and  Imperial  Gage),  are  rarely  found  planted  with  it.  As  a 
pollenizer  it  has  proved  effective  on  the  French,  but  because  of  an 
improperly  timed  operation  in  the  one  year  it  was  tried,  it  was  not 
so  successful  on  the  Sugar.  In  view  of  the  foregoing  it  would  cer- 
tainly seem  advisable  to  interplant  Robe  de  Sergeant  prunes  with 
either  the  French  or  Sugar  varieties. 

Review  of  the  Results  Obtained  in  the  Santa  Clara  Valley. — The 
most  interesting  fact  noticed  in  studying  the  set  of  the  fruit  under 
open  orchard  conditions  in  two  Santa  Clara  Valley  orchards  was  the 
small  percentage  of  blossoms  which  matured  fruit.  Yet  it  is  with 
these  low  percentages  of  set  that  the  commercial  crop  of  prunes  of 
California  is  produced.  In  the  Pettit  orchard  at  Cupertino  this  set 
has  been  remarkably  uniform  and  has  yielded  each  year  what  the 
owner  considers  to  be  a  fair  crop.  In  the  Sorosis  orchard  at  Saratoga 
the  set  was  fairly  uniform  for  1915  and  1916,  but  in  1917  the  owner 
placed  some  115  colonies  of  bees  in  the  orchard  during  the  blossoming 
season.  Because  of  the  bees  the  percentage  of  set  increased  greatty 
in  1917  and  raised  the  average  for  the  three-year  period  up  to  8.3 
per  cent.4  Moreover,  the  yield  emphasizes  the  effective  work  of  the 
bees.  In  1916  with  the  normal  set  averaging  3.2  per  cent  the  yield  on 
180  acres  was  344  tons  of  dried  prunes.  On  the  same  acreage  in  1917 
with  an  average  set  of  approximately  12.9  per  cent  the  yield  was  432, 
or  an  increase  of  nearly  100  tons  of  dried  fruit.  It  is  interesting  to 
contrast  the  small  percentage  of  normal  set  in  the  Santa  Clara  Valley 
with  the  high  percentages  of  normal  set  on  the  younger  trees  at  the 
University  Farm. 

One  of  the  chief  problems  confronting  the  grower  of  the  Imperial 
variety,  is  how  to  get  it  to  yield  more  heavily.  Its  value  for  a  fancy 
dried  prune  cannot  be  surpassed  as  far  as  quality  and  flavor  are 

4  For  full  account  see  Cal.  Exp.  Sta.  Bull.  291. 


24  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

concerned,  but  the  tree  except  in  a  few  localities,  has  persistently 
refused  to  bear  profitably.  In  the  Pettit  orchard  the  normal  set  has 
held  consistently  between  3  and  4  per  cent,  averaging  3.5  per  cent  in 
the  last  three  years.  Crops  resulting  from  these  percentages  of  set 
were  not  especially  heavy.  In  the  Sorosis  orchard  the  normal  set  for 
four  years  average  7.25  per  cent.  The  resultant  yield  each  year  was 
considered  good,  and  the  orchard  has  a  reputation  for  heavy  yields 
as  far  as  this  variety  is  concerned.  Although  the  actual  percentages 
of  normal  set  for  the  years  1916,  1917,  and  1918  were  low,  due  prob- 
ably to  the  comparatively  small  number  of  trees  used  in  making  the 
count,  nevertheless  a  somewhat  larger  crop  was  harvested  in  1917  than 
the  year  before,  due  to  the  work  of  bees. 

The  Sugar  prune  trees  in  the  Pettit  orchard  averaged  under 
normal  conditions  a  set  of  11.7  per  cent.  Although  this  is  a  com- 
paratively high  percentage,  the  scarcity  of  blossoms  made  the  resultant 
yields  rather  below  what  might  be  considered  a  good  crop.  The  Sugar 
prune  does  not  blossom  so  profusedly  as  either  the  French  or  the 
Imperial.  Again  the  normal  set  in  the  Santa  Clara  Valley  was  lower 
than  on  trees  of  the  same  variety  at  Davis.  Because  of  the  general 
scarcity  of  blossoms  on  the  Sugar  it  is  doubtful  whether  the  average 
percentage  of  set  obtained  in  the  Pettit  orchard  is  high  enough  to  be 
commercially  profitable. 

There  was  but  little  doubt  left  at  the  end  of  three  years  as  to  the 
self-fertility  of  the  French,  Sugar,  and  Imperial  prunes  as  grown  in 
the  Santa  Clara  Valley.  Results  in  1918  tend  strongly  to  confirm 
these  results.  The  French  in  both  orchards  has  consistently  proved 
to  be  self -fertile.  This  fact  is  so  plainly  evident  that  further  discus- 
sion is  unnecessary. 

Except  for  one  year  the  Imperial  has  proved  to  be  self-sterile,  or 
practically  so.  In  1917,  in  the  Sorosis  orchard,  a  set  of  5.3  per  cent 
was  obtained.  In  the  Pettit  orchard  with  the  same  variety  in  1916 
about  1  per  cent  set  resulted.  At  Sorosis  it  was  absolutely  self -sterile 
in  1918. 

The  set  of  5.3  per  cent  in  1917,  together  with  the  fact  that  another 
Imperial  tree  when  enclosed  in  a  tent  with  bees  matured  a  crop  of 
3  per  cent,  shows  that  at  times  this  variety  is  probably  capable  of 
self-fertilization.  One  year's  satisfactory  results,  out  of  four,  how- 
ever, is  far  too  small  a  proportion  to  warrant  depending  entirely 
upon  self-pollination  to  set  a  crop.  Neither  does  practical  experience 
warrant  it.  For  the  purpose  of  the  general  fruit  grower  it  must  be 
considered  self -sterile  and  careful  steps  taken  to  bring  about  heavier 
production  by  interplanting  with  a  good  pollinator. 


PLUM    POLLINATION  25 

Investigations  as  to  the  probable  cause  of  self-sterility  of  this 
variety  have  not  as  yet  gone  beyond  the  stage  of  observation.  A 
number  of  peculiarities  have  been  noticed,  each  of  which  may  have 
some  bearing  on  the  problem.  The  Imperial  is  perhaps  the  most 
prolific  bloomer  of  all  the  varieties  of  European  plums  commonly 
found  in  California.  The  blossoms  are  tightly  packed  in  great  pro- 
fusion on  rather  short,  rigid  spurs.  Often  a  comparatively  large 
proportion  of  these  blooms  appear  weak ;  the  petals  are  small  and  far 
apart,  and  the  stamens  and  pistils  are  short.  Quite  often  the  flower 
does  not  open  properly,  but  allows  some  of  the  outer  floral  parts  to 
remain  folded  over  the  sexual  organs.  The  stem  is  very  short  and  the 
whole  flower  often  drops  easily  even  when  touched  lightly.  The 
remainder  of  the  flowers  seem  normal  in  every  respect  and  are  pro- 
duced on  a  long  stem.  Another  peculiarity  was  noticed  about  the 
Imperial  pollen.  It  germinated  slowly  and  made  a  very  short  growth, 
probably  not  more  than  one-fifth  as  long  as  the  tubes  put  out  by 
either  the  Sugar  or  French  varieties.  In  many  cases  the  tube  was 
hardly  more  than  a  fairly  distinct  bulge  on  the  side  of  the  pollen 
grain.  Germination  counts  for  this  variety  were  often  made  twelve 
or  twenty-four  hours  later  than  with  the  other  varieties  to  obtain 
tubes  long  enough  to  be  readily  distinguished. 

The  Sugar  prunes  were  found  to  be  able  to  set  fruit  with  their 
own  pollen.  This  evidence  supports  the  data  obtained  at  the  Univer- 
sity Farm.  During  the  season  of  1918,  the  yield  obtained  from  self- 
pollinated  flowers  was  the  best  of  the  four  years,  and  practically  the 
same  as  the  yields  obtained  from  either  the  Imperial  or  French  crosses. 

Careful  work  for  three  years  failed  to  show  any  lack  of  affinity 
between  any  of  the  three  varieties  of  prunes  principally  grown  in  the 
Santa  Clara  Valley.  Occasionally  a  particular  cross  in  one  or  the 
other  of  the  orchards  used,  failed  to  mature  a  crop.  This  was  particu- 
larly true  the  first  season  in  the  Sorosis  orchard,  because  of  the 
extremely  warm  weather.  The  same  crosses  in  the  Pettit  orchard 
made  a  day  or  two  earlier  were  eminently  successful.  Since  the  first 
year  nearly  all  the  cross-pollinations  made  have  responded  positively, 
except  in  1917  when  the  Sugar  and  Imperial  crossed  with  French 
failed  to  set.  The  Imperial  X  French  cross,  however,  was  successful 
at  Sorosis,  and  the  Sugar  X  French  cross  in  1918  was  also  successful. 
Chief  value  should  be  attached  to  the  1917  results,  as  they  were 
obtained  by  using  a  very  large  number  of  each  cross. 

The  French  prune  at  both  places  showed  a  remarkably  high 
average  set  when  crossed  with  either  Imperial  or  Sugar.  As  a  pollen - 
izer  for  the  other  two  varieties,  it  did  not  average  high  during  the 


26  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

three  years,  but  nevertheless  served  efficiently.  The  1918  results  tend 
to  bring  up  this  average.  With  the  Imperial,  the  crosses  made  by 
hand  did  not  average  as  high  as  the  normal  set,  but  from  the  excellent 
results  obtained  in  individual  years,  this  variety  was  shown  to  be 
easily  pollinated  by  the  other.  As  a  pollenizer  it  averaged  higher 
than  the  French.  The  Sugar  proved  capable  of  being  pollinated  by 
both  Imperial  and  French  and  likewise  served  as  an  efficient  pollenizer 
for  them.  On  the  strength  of  the  foregoing  facts  there  can  be  no 
doubt  about  the  value  of  interplanting  these  varieties  for  cross- 
pollination.     The  Imperial,  especially,  needs  this  cross-pollination. 

Careful  observation  showed  that  crossing  the  varieties  produced 
no  effect  on  the  size  of  the  fruit  produced.  This  was  true  even  when 
Imperial  pollen  was  used  on  a  small  fruited  variety  like  the  French. 
Neither  were  there  any  noticeable  effects  on  the  shape  of  the  fruits  as 
a  result  of  the  cross-pollination  (figs.  4  and  5). 

CONCLUSIONS 

1.  The  following  varieties  of  Japanese  plums  were  found  to  be 
self -sterile  at  the  University  Farm  at  Davis,  California :  Combination, 
Kelsey,  Satsuma,  Burbank,  Wickson,  Sultan,  and  Abundance. 

2.  Climax,  another  Japanese  variety,  is  apparently  self -fertile. 

3.  In  general,  the  early  blooming  Japanese  varieties  such  as  Com- 
bination, Kelsey,  and  Satsuma,  are  scanty  pollen  producers  and  not 
effective  pollenizers,  while  the  later  blossoming  varieties  such  as  Bur- 
bank,  Wickson,  Climax,  Sultan,  and  Abundance  produce  pollen 
abundantly  and  are  effective  pollenizers. 

4.  Clyman,  Tragedy,  and  probably  Grand  Duke,  are  self-sterile, 
while  Yellow  Egg  and  Pond  from  one  year's  data  appear  self-fertile. 
Pond,  or  Hungarian  Prune,  as  it  is  known  in  California,  has  long  been 
popularly  regarded  as  self-sterile,  so  too  much  weight  should  not  be 
given  to  the  results  obtained  in  one  year. 

5.  All  of  the  five  European  varieties  tested  seemed  to  be  abundant 
pollen  producers  and  to  cross-pollinate  one  another  effectively. 

6.  The  French  and  Sugar  prunes  are  self-fertile  and  therefore 
need  not  be  interplanted  with  other  varieties  to  secure  satisfactory 
yields.  It  is  probable,  however,  that  their  yield  may  be  increased, 
especially  in  certain  years,  if  interplanting  is  practiced. 

7.  Imperial  and  Robe  de  Sergeant  prunes  must  be  considered  self- 
sterile  for  practical  purposes. 

8.  Sugar,  Imperial,  and  French  prunes  cross-pollinate  one  another 
effectively.  Robe  de  Sergeant  was  found  to  cross-pollinate  readily 
with  Sugar  and  French,  but  was  not  tested  with  Imperial. 


PLUM    POLLINATION  27 

9.  No  evidence  of  inter-sterility  among  plums  or  prunes  was  found. 

10.  A  comparison  over  a  number  of  years  between  trees  where  there 
was  an  abundance  of  bees  flying  and  trees  where  bees  were  scarce, 
emphasized  the  desirability  of  having  bees  in  the  orchard.  Even  self- 
fertile  varieties  were  immenseh7  benefited  by  the  presence  of  bees  as 
an  agency  for  distributing  the  pollen. 

11.  Climatic  factors  are  probably  important  in  the  setting  of  plums 
and  prunes  as  evidenced  by  the  fluctuating  set  obtained  with  certain 
crosses  in  succeeding  years.  Investigations,  however,  have  not  pro- 
gressed far  enough  as  yet  to  throw  any  light  upon  this  phase  of  the 
problem. 

12.  The  following  is  a  list  of  self -fertile  and  self-sterile  plums  and 
prunes  as  found  by  the  Division  of  Pomology  of  the  University  of 
California. 


Self-sterile 

8  elf -fertile 

Doubtful 

Combination 

Climax 

Pond 

Kelsey 

French 

Satsuma 

Sugar 

Burbank 

Wickson 

Sultan 

Abundance 

Clyman 

Tragedy 

Yellow  Egg 

Grand  Duke 

Imperial 

Robe  de  Sergeant 

ACKNOWLEDGMENTS 

The  author  was  assisted  at  various  times  by  R.  R.  Holeman,  H.  A. 
Houston,  F.  M.  Croce,  E.  S.  Yocco,  and  W.  O.  Dessauer,  who  as  mem- 
bers of  succeeding  classes  in  pomology  worked  on  special  phases  of 
the  pollination  problem  for  their  theses.  In  the  Santa  Clara  Valley 
the  former  Horticultural  Commissioner,  Mr.  Earl  Morris,  as  well  as 
the  present  Commissioner,  Mr.  R.  F.  Cody,  gave  material  assistance. 
The  author  takes  this  opportunity  of  expressing  his  appreciation  and 
thanks  for  the  aid  rendered  by  these  men.  Thanks  are  also  due  to 
Mr.  E.  F.  Pettit  of  Cupertino,  Mr.  Frank  Abernathy,  manager  of  the 
Sorosis  Fruit  Company,  and  Mr.  Millard  Sharpe  of  Vacaville,  who 
placed  their  orchards  at  his  disposal.  The  writer  is  indebted  to 
Dr.  W.  L.  Howard  of  the  University  Farm  for  his  painstaking  care 
in  correcting  and  revising  the  manuscript,  as  well  as  for  much  helpful 
advice  in  the  course  of  the  investigations. 


28  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


LITERATURE   CITED 

Waugh,  F.  A. 

The  Pollination  of  Plums.     10th,  11th,  12th,  and  13th  reports  of  the  Vermont 
Agric.  Exp.  Sta.,  1896,  1897,  1898,  1899. 

Gardner,  V.  E. 

Pollination  of  the  Sweet  Cherry.     Oregon  Agric.  Exp.  Sta.  Bull.  116,  1913. 

Tufts,  W.  P. 

Almond  Pollination.     Calif.  Agric.  Exp.  Sta.  Bull.  301,  1919. 

Lewis,  C.  I.,  and  Vincent,  C.  C. 

Pollination  of  the  Apple.     Ore.  Agric.  Exp.  Sta.  Bull.  104,  1909. 


